The blades of conventional airscrews are normally secured to the hub through blade necks which are symmetrically distributed around the hub. More specifically, the angular spacing between the radial axes of the blade necks are equal to one another. The longitudinal axes of the blades also extend radially and coincide with the radial blade neck axes. Hence, the blades are also equally spaced from one another around the hub.
Airscrews rotating at high speeds, as is the case for propellers or fan drives, produce during their operation a noise level that depends on the circumferential speed of the airscrew and on the flight speed of the aircraft. Additionally, the noise level is influenced by or depends on the on-flow conditions, as well as the propeller blade configurations and on the loads to which the propeller blade is exposed. An especially irksome noise component is the so-called rotating noise of the airscrews since it involves higher frequencies which are subjectively noted more intensely than lower frequency noise components. This so-called rotating noise depends on the r.p.m. and on the number of blades in the airscrew.
Frequently, the r.p.m. and the number of blades are given values that are dictated by structural considerations. Thus, it is not normally possible to sufficiently change these values for positively influencing the rotating noise. Another aggravating factor is the amplification of the rotating noise by wake or trailing effects caused by obstructions in the on-flow of the airstream. Such obstructions can be struts or fairing elements of an aircraft and this amplification takes place each time when a blade moves past such obstructions.
It has been suggested for reducing the noise caused by exhaust fan wheels to arrange the wheel blades including their blade necks at differing pitches. In other words, the angular spacing between neighboring blades differs around the hub. As a result, it takes different time durations for the blades of such an exhaust fan rotor to pass by fixed points of the fan structure, thereby causing a frequency shift. Such a construction, however, is suitable only when blade angle adjustments, that is adjustments of the angle of attack of the blade, are not necessary. Such blade angle adjustments are, however, necessary, e.g. due to large changes in the on-flow speed of the air. These blade angle adjustments are required, for example, in connection with conventional propellers and so-called jacketed airscrews. The blade angle adjustments require involved mechanisms that need a certain volume within the screw hub for their location. Such volume needs cannot be ignored. Additionally, the adjustment mechanisms require respective adjustment forces for the operation of the mechanism which also imposes limitations on the construction of such adjustment mechanisms even when the blade necks are distributed symmetrically around the hub, that it, with equal angular spacings. A non-symmetric hub pitch for the individual blade necks would complicate the blade angle adjustment mechanism to such an extent that it could not be realized in any economically feasible way.